It is believed the taxi, a Hackney carriage, picked up the victim in the area of Bishopsgate, in the City of London, at around 7pm before dropping him off in Byng Street shortly before 7.30pm.

The spokesman added: "Officers are now keen to speak with the driver as they may have been in the area when the shooting took place. Police were called at approximately 7.30 to reports of the shooting."

Mr Gorbuntsov was shot a number of times as he entered a block of flats by a suspect who is described as white, 6ft tall and of slim build.

He was wearing a dark hooded top and seen running from Byng Street away from Westferry Road, towards Marsh Wall.

Detectives, who are treating the shooting as attempted murder, are keen to speak to anyone who was in the area at the time or may have information about the incident.

The victim, in his 40s, previously owned banks in Russia and Moldova, according to Russian media.

Police would not confirm Russian reports that the attacker had used a sub-machine gun and officers would not say where Mr Gorbuntsov is being treated.

A source said the victim was under armed guard.

Russia's Kommersant newspaper quoted Mr Gorbuntsov's lawyer, Vadim Vedenin, who said his client had submitted evidence to police investigating the attack on Mr Antonov in 2009.

Three Chechen men were convicted of attempted murder in the case and given long jail sentences in 2010.

The apparent attempted assassination will prompt comparisons with the murder of Russian dissident Alexander Litvinenko in London in November 2006.

British prosecutors have named fellow ex-KGB agent Andrei Lugovoy as the main suspect in his poisoning with radioactive polonium-210, but the Russian authorities have repeatedly refused to send him to face trial in the UK.